Method for detecting the length of sheet in a printer

Abstract

In the determination of the length of a sheet that has been inserted in a printer, an optical sensor scans the length of the sheet as it is fed into the printer, and a counter is controlled to provide a count corresponding to the length of the sheet. When the sensor encounters a mark on the sheet, such as a line, it provides an output indicating that the sheet is not present. This erroneous output is corrected by moving the sensor laterally of the sheet to a position displaced from the mark to enable further sensing of the presence of the sheet in a region that does not include the mark.

What I claim is: 
     
       1. In the method for detecting the length of a sheet in a printer, comprising the step of detecting the sheet length with an optical sensor while advancing the sheet in the lengthwise direction of the sheet in the printer, the improvement comprising moving said sensor in a direction transverse to the lengthwise direction of the sheet in the printer to steer clear of undetectable portions of said sheet, and detecting the length of the sheet at a position clear of said undetectable portions. 
     
     
       2. The method of claim 1, wherein said optical sensor is mounted on a carrier of a printing head, said step of moving comprising moving said carrier. 
     
     
       3. A method for determining the length of a sheet as it is fed in a printer, the sheet having side edges and the printer having means for advancing the sheet in the lengthwise direction of the sheet in the printer, wherein the printer has a sensor for detecting the presence or absence of a sheet, said method comprising positioning said sensor at a first position between said side edges of said sheet, and sensing the presence and absence of said sheet with said sensor while said sensor is at said first position and while said sheet is being advanced in said lengthwise direction, stopping said sheet and displacing said sensor from said first position to a second position toward one of said side edges upon the sensing of the absence of said sheet by said sensor, and the advancing said sheet and sensing the presence and absence of said sheet with said sensor while said sensor is at said second position. 
     
     
       4. The method of claim 3 further comprising, upon sensing of the absence of said sheet at said second position, stopping said sheet and displacing said sensor from said second position to a third position toward the other side edge of said sheet, and then advancing said sheet and sensing the presence and absence of said sheet with said sensor while said sensor is at said third position. 
     
     
       5. The method of claim 3, wherein said step of positioning said sensor at a first position comprises moving said sensor to a center point between said side edges of said sheet. 
     
     
       6. The method of claim 3, wherein said step of displacing said sensor to a second position comprises moving said sensor to a home position at the left margin of said sheet. 
     
     
       7. The method of claim 3 comprising stepping a counter as said sheet is advanced, whereby said counter stores a count corresponding to the advance of said sheet, and stopping the stepping of said counter while said sensor is moved from said first position to said second position.